1. Technical Field
The embodiment described herein relates to a communication system and, more particularly, to an orthogonal frequency division multiplexing (hereinafter, referred to as ‘OFDM’) system and a data transmission method therefor.
2. Related Art
Since an OFDM system, which is suitable for high-speed data transmission in a wired and wireless channel, uses a plurality of subcarriers having inter-orthogonality, the OFDM system efficiently uses a frequency. A process of modulating and demodulating a plurality of carriers in a transmitter and a receiver has the same result as performing inverse fast Fourier transform (IFET) and fast Fourier transform (FET), thereby implementing high-speed data transmission and reception.
The OFDM system is adopted in various wireless communication applications such as digital audio broadcasting (DAB), terrestrial digital video broadcasting (DVB-T), European Telecommunications Standard Institute (ETSI) HIPERLAN/2 standard, Institute of Electrical and Electronics Engineers (IEEE) 802.11a standard for a wireless local area network (WLAN), IEEE 802.16a for a wireless metropolitan access network (WMAN), etc.
The OFDM system is high in spectral efficiency. The OFDM system can digitally implement modulation and demodulation units with efficiency and transmit data at high speed. On the contrary, a peak-to-average power ratio (hereinafter, referred to as ‘PAPR’) is high.
Most of wireless communication systems including the OFDM system use a high power amplifier (HPA) in order to acquire sufficient transmission power in the transmitter. In order to acquire peak output power from the HPA, an operating point must be established in the vicinity of a saturation region. As a result, the HPA has non-linear characteristics. Inter-modulation distortion and distortion of an undesirable band due to the non-linear characteristics of the HPA cause the PAPR to be increased. Moreover, the distortions and radiation cause high interference between adjacent channels as well as increasing a bit error ratio (BER).
In the OFDM system, the PAPR can represent a ratio of peak instantaneous power to average power of a transmission signal. General PAPR reducing methods adopt a method of reducing the peak instantaneous power. The method of reducing the PAPR can include a clipping method, a block coding method, a phase adjusting method, etc. Recently, as the method of reducing the PAPR, a selective mapping (SLM) method which is one of the phase adjusting methods has been widely used.
FIG. 1 is a block diagram illustrating a structure of an OFDM transmitter using an SLM method.
As shown in the FIG. 1, input data to be transmitted is divided into a plurality (M−1) of sequences in a serial/parallel transformer (S/P) 10. The divided sequences ‘X(k)’ are inputted into a phase rotating unit 12 and coupled with phase sequences ‘φm(k)’ (m=0, 1, . . . , M−1) so as to be generated as OFDM sequences ‘Xm(k)’ of a frequency domain which are independent from each other. The OFDM sequences ‘Xm(k)’ of the frequency domain re inputted into an inverse fast Fourier transformer 14 so as to be generated as OFDM sequences ‘xm(n)’ of a time domain. Thereafter, the OFDM sequences ‘xm(n)’ of the time domain are inputted into a minimum PAPR selection unit 16. Herein, any one sequence ‘χm−(n)’ having the smallest PAPR is selected and transmitted.
As described above, in the SLM method, the OFDM sequences are rotated by multiplexing of plural complex numbers and IFFT computation. IFFT modules are needed as many as the sequences. As the number of carriers increases, the number of IFFT modules and a computing quantity in the IFFT modules increase. Consequently, complexity of the system increases, thereby deteriorating the performance of the system.